Generally a hydraulic stepless change speed apparatus (Hydrostatic power transmission) used in a tractor includes a charge oil line to maintain a full supply of actuating oil in this apparatus, and pressure oil is fed thereto from a charge pump. According to the prior art, this charge pump is attached to the hydraulic stepless change speed apparatus and, when the main clutch is put out, the charge pump is brought out of operation stopping the pressure oil supply for charging. Therefore, the supply of pressure oil for charging is interrupted while the main clutch is put out in order to effect a gear change to the gear transmission or stop operation of the working implement, and the main clutch is frequently put out depending on the type of gear changing or working implement which, although temporarily, results in a loss in transmission efficiency due to shortage of the pressure oil for charging.
Further, tractors are sometimes equipped with means to retain the main clutch disengaged in order to prevent transmission faces of the main clutch from sticking to each other by rusting or the like during a long period of nonuse. With the main clutch so kept disengaged, even if the engine is started, the hydraulic stepless change speed apparatus is not filled with the charge oil and this change speed apparatus cannot be driven properly without waiting for a while after putting in the clutch. In this case, since the vehicle starts moving upon putting in the main clutch although the drive is uncertain, the driver generally confirms prior to putting in the main clutch that the change speed operating device is in neutral in order to prevent the vehicle from inadvertently moving, and thereafter puts in the main clutch, wherefore it tends to take a long time from the start of the engine till the start of the vehicle with the hydraulic stepless change speed apparatus working properly.
As one means of shortening the preparation time for driving the hydraulic stepless change speed apparatus, it has been devised, for example, to increase the capacity of the charge pump and at the same time use a specially provided accumulator so that charging is effected in a short time after the main clutch is put in, or to provide an additional charge pump on the engine side so that charging starts simultaneously with the start of the engine, but then the construction becomes complicated and besides the apparatus space for obtaining a charge oil pressure source becomes large, making it hardly applicable to small tractors.
The object of the present invention is to provide for supply of charge pressure oil to the hydraulic stepless change speed apparatus to be carried out reliably at all times and prevent the occurrence of inconvenience as noted above, by a hydraulic circuit reconstruction to effectively utilize the existing hydraulic pump for supplying pressure oil to the hydraulic circuit for the lift arm cylinder for raising and lowering the attached working implement.